1. Field of the Invention
The present invention relates generally to data transmission, and more particularly to routing of data between different data transmission protocols.
2. Description of Related Art
Communication of data between devices over a network conventionally adheres to the Open Systems Interconnection (OSI) network model. The OSI network model is generally viewed as a stack of seven layers, each layer having protocols that support exchange of data over the layer.
FIG. 1 illustrates a generalized diagram of an OSI network model 100 found in the prior art. OSI network model 100 includes seven layers: layer 1, the physical layer; layer 2, the data link layer (further including a Media Access Control (MAC) layer and a Logical Link Control (LLC) layer); layer 3, the network layer; layer 4, the transport layer; layer 5, the session layer; layer 6, the presentation layer; and, layer 7, the application layer. OSI network model 100 is well known to those of skill in the art and not further described in detail herein to avoid detracting from the present invention.
Synchronous Optical Network (SONET) protocol is the version of the transmission standard published by the American National Standards Institute (ANSI) utilized by the United States. SONET encompasses all of the physical layer protocol (layer 1) and parts of the media access control layer protocol (layer 2) that frame data for synchronous data transmission over fiber optic networks. Transfer of information over SONET is primarily implemented using two data type formats, cells or packets. Cells are pieces of data having a fixed size, and are conventionally transmitted over SONET using Asynchronous Transfer Mode (ATM) Over SONET protocol. Packets are pieces of data not fixed in size, and are conventionally transmitted over SONET using Packet Over SONET (POS) protocol that utilizes a layer 2 protocol called Point-to-Point Protocol (PPP).
Frequently, data transmitted over a fiber optic line is destined for a receiving device on a local area network (LAN) of twisted pair or coaxial cable that utilizes an Ethernet protocol. Thus, the data must be transferred from the fiber optic network utilizing SONET to the LAN that utilizes Ethernet.
Typically, a switch element, such as a router, is involved in communicating information between SONET networks and Ethernet networks. The switch element is involved in translating the SONET frames to Ethernet frames, and as part of that translation needs to convert the layer 1/layer 2 parts of those protocols from SONET to Ethernet. The layers 3 through 7 parts of the protocol remain intact. While the conversion of ATM Over SONET protocol to Ethernet protocol has been standardized, the conversion from POS protocol (e.g., PPP) to Ethernet protocol has not.
FIG. 2A illustrates a generalized diagram of a PPP frame 200 from a POS frame found in the prior art. PPP frame 200 is shown including a PPP identifier (PPP ID) value 210 and a data value 212. Data value 212 includes the actual data that are being transferred from a source computing entity to a destination computing entity.
FIG. 2B illustrates a generalized diagram of an Ethernet frame 220 found in the prior art. Ethernet frame 220 includes: a Media Access Control destination address (MAC DA) value 222; a MAC source address (MAC SA) value 224; an Ethernet Virtual Local Area Network (VLAN) tag value 226, composed of an Ethernet VLAN tag header value 236 and an Ethernet VLAN tag identifier (ID) value 238; an Ethertype value 228; a data value 230; pad values 232, if needed; and an FCS value 234, e.g., a cyclic redundancy check (CRC) value. Items 222, 224, 226, 236, 238, 228, 232, and 234 are all parts of the Ethernet layer 2 protocol. Data value 230 includes the layer 3 through 7 headers and actual data that are being transferred from a source computing entity to a destination computing entity.
In both PPP frame 200 and Ethernet frame 220, there is an identifier that specifies the layer 3 protocol. In PPP frame 200, the identifier is PPP identifier (PPP ID) value 210, however, in Ethernet frame 220, Ethertype value 228 identifies the layer 3 protocol type found in data field 230. Thus, a switch element involved in communicating data value 212 between a POS and an Ethernet network needs some means for passing the layer 3 protocol information provided by PPP ID value 210 to a receiving switch element in order for the switch element to make an efficient routing decision.